ci: use separate docker images for baremetal builds

Using arm_test-base as a separate base layer as well for storage &
network bandwidth efficiency.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5381>
6 files changed